1. Field of the Invention
The invention relates to narrow band lasers and particularly to an excimer or molecular fluorine laser having output coupling interferometer.
2. Discussion of the Related Art
Narrow band excimer lasers (xcex=193 nm, 248 nm) are applied in photolithographic applications for production of integrated circuits. Excimer laser radiation is used for making structures in the dimensional range of  less than 0.18-0.25 xcexcm (KrF-laser radiation) or  less than 0.13-0.18 xcexcm (ArF-laser radiation). The molecular fluorine laser emitting around 157 nm (F2-laser) is being developed for feature sizes  less than 0.13 xcexcm. Achromatic imaging optics are difficult to produce for this wavelength region. For this reason radiation of narrow bandwidth is desired to control imaging errors caused by chromatic aberration. Acceptable bandwidths are typically less than 0.6 pm.
Another important beam parameter is the spectral purity, or the bandwidth which contains 95% of the output pulse energy. High numerical aperture (NA) optics use  less than 1 pm bandwidth radiation. This can achieved by using of two spectral narrowing elements such as a grating and intracavity etalon or etalon output coupler.
Etalon outcoupling mirrors have been used for a long time and in various different types of lasers. A simple example of a planexe2x80x94plane cavity for an excimer laser can be formed by a highly reflective (HR) back-mirror and an uncoated solid etalon as an outcoupling resonator reflector.
U.S. Pat. Nos. 5,901,163 and 5,856,991 each to Ershov relate to a resonator including an etalon output coupler for a narrow band excimer laser, as shown in FIG. 1 (which is FIG. 3 of the ""991 patent). The resonator consists of a line narrowing module (18) consisting of an echelle grating and a prism beam expander, and a plane-parallel air spaced etalon (44) as an outcoupling mirror.
The echelle grating based line narrowing module produces a laser beam having a spatial variation in wavelength (chirp) along a beam cross section direction (direction of dispersion). FIG. 2 shows a typical spatial distribution of a laser spectrum across the beam created by the grating. The laser resonator used for generating the spectrum in FIG. 2 consists of an echelle grating, prism beam expander and a typical partially reflecting outcoupling mirror having a reflectivity of, e.g., 20-25%.
Thus, for the arrangement of FIG. 1, the line narrowing module (18) provides a spatial distribution of wavelengths at the outcoupling etalon that is approximately given by:
xcex(x)=xcex(0)+(dxcex/dx)xxe2x80x83xe2x80x83(equation 1); 
where x is the coordinate along the short beam axis, and x=0 is the beam center. For the example depicted in FIG. 2, the xe2x80x9cspatial chirpxe2x80x9d is dxcex/dx≈0.83 pm/mm. This value depends on the linear dispersion of the echelle grating and the laser design (i.e., the distance between the grating and outcoupling etalon, the discharge width, etc.).
FIGS. 3a, 3b show two calculated spatial distributions of laser spectra for two different gratings (dxcex/dx=0.83 pm/mm and 1.24 pm/mm), an airspaced plane-parallel uncoated etalon with FSR=1.6 pm as outcoupler and otherwise the same resonator designs. FIG. 3c shows the measured spectrum for a grating with dxcex/dx=1.24 pm/mm and an outcoupler etalon with FSR=1.6 pm. The calculations are in a good agreement with the experimental findings (i.e., compare FIGS. 3b and 3c).
To avoid xe2x80x9cside modesxe2x80x9d the following relation is fulfilled:
(dxcex/dx)xc2x7bxe2x89xa60.5 FSRxe2x80x83xe2x80x83(equation 2); 
where b is the beam width in front of the etalon. Higher values for dxcex/dx can be achieved by using more highly dispersive gratings, or bending the grating such as is disclosed in U.S. Pat. No. 5,095,492 to Sandstrom. As it is desired to produce still smaller structures on silicon substrates, it is desired to further reduce the spectral purity of excimer laser exposure beams.
It is therefore an object of the invention to provide a narrow band excimer or molecular fluorine laser having improved spectral purity.
In accordance with this object, an excimer or molecular fluorine laser is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator including the discharge chamber and a pair of resonator reflectors for generating an output laser beam. One of the resonator reflectors is an output coupling interferometer including a pair of opposing reflecting surfaces tuned to produce a reflectivity maximum at a selected wavelength for narrowing a linewidth of the output laser beam.
In a first aspect of the invention, one of the pair of opposing reflecting surfaces is configured such that the opposing reflecting surfaces of the interferometer have a varying optical distance therebetween over an incident beam cross-section which serves to suppress outer portions of the reflectivity maximum to reduce spectral purity. Preferably, this surface is non-planar, and may include a step, a recess or a raised or recessed curved portion of a quarter wavelength in height or depth, respectively.
In a second aspect of the invention, the laser includes a first photodetector and a beam splitter. The beam splitter is positioned to reflect a portion of the beam reflected from the output coupling interferometer to the photodetector. The interferometer is tuned substantially to a maximum intensity of interference fringes reflecting therefrom. Preferably, a second photodetector and a second beam splitter are positioned to monitor the beam transmitted through the output coupling interferometer. Information detected at the second photodetector is used by a processor for maximizing an energy stability of the transmitted beam.
In a third aspect of the invention, an etalon spectrometer is positioned to detect spectral information of the beam transmitting through the output coupling interferometer. The output coupling interferometer is tuned to produce a maximum intensity of interference fringes of the etalon spectrometer.
In a fourth aspect of the invention, a position sensitive photodetector and a beam splitter are included. The beam splitter is positioned to reflect a portion of the beam reflected from the output coupling interferometer to the position sensitive photodetector. The output coupling interferometer is tuned substantially to a maximum intensity of reflection interference fringes. Preferably, a second photodetector and a second beam splitter are also included, wherein information is detected at the second photodetector of the beam transmitted through the interferometer and used by a processor for maximizing energy stability of the transmitted beam.
In a fifth aspect of the invention, the output coupling interferometer is disposed within a housing. A pressure control unit controls a pressure within the housing and between the first and second opposing reflecting surfaces of the interferometer. The pressure control unit preferably included an inert gas filled bellows fluidly coupled with the housing. The interior volume of the bellows is adjustable for adjusting the pressure within the housing and between the first and second opposing reflecting surfaces of the output coupling interferometer.
In a sixth aspect of the invention, a beam expander is disposed before the output coupling interferometer. The beam expander reduced the divergence of the beam incident at the interferometer, the resolution of the interferometer is improved, and the spectral purity is improved in accord with the object of the invention. The beam expander may include one or more beam expanding prisms or a lens arrangement.